1. Field of the Invention
This invention relates to data link acquisition and reacquisition systems. More specifically, the present invention relates to a data link system between two stations, platforms or aircraft both of which are rapidly moving relative to each other during periods of communication outage and the invention concerns an optimum method of reacquisition of the spread spectrum code signals.
2. Description of the Prior Art
This invention is an improvement over our two aforementioned related pending applications and is concerned with optimizing or reducing the reacquisition time after short periods of outage. The two above mentioned applications show and claim apparatus and method for reducing acquisition time after long outages. Our former application employs a system for advancing the spread spectrum coded signals being transmitted from the transmitting station relative to a reference clock by a predetermined first time vector. The receiving portion of the system employs means for retarding the replica spread spectrum coded signals being received relative PG,3 to the same reference clock by a predetermined second time vector. The present invention permits both stations to independently advance or retard their transmitted coded signals and also the replica code generators based on a range approximation when the receiving and transmitting stations are pointing their directional antennas directly at each other, thus, reducing the window of search time to a minimum during sequential attempts to achieve lock-on or acquisition at different search angles or positions of the antenna.
Our latter application employs a method of pointing the receiving and the transmitting antennas on equal and inverse pointing directions and then preceding to search by moving the direction of the antennas toward each other. As the antennas are moved to sequential cell positions each cell position is held during an acquisition attempt knowing that the equal and opposite movement of the antenna positions will eventually result in the receiving and transmitting antennas pointing exactly toward each other. The latter application offers a great improvement in acquisition time after long outages. The present invention further improves acquisition time by reducing the number of cells to be searched after short outages, thus, reducing reacquisition time which occurs after a previous acquisition had been achieved. The present invention improves the acquisition time required for both previous inventions and is extremely useful in situations where the platform, station or aircraft is making an intelligence gathering mission over unfriendly territory.
The present invention makes it possible to search fewer chips in a long sequence pseudonoise spread spectrum code so as to achieve reacquisition between a fast moving platform and a home station more rapidly than was heretofore achievable when there is a short outage.